AGE-RELATED-CHANGES IN RAT COLON MECHANICS

To determine whether myogenic factors are responsible for slowed colon ic transit in senescent rats, maximum shortening velocity (V0), compli ance of the series elastic component (SEC), measurements of passive fo rce in calcium-depleted tissue and peak isometric force (F0) were exam ined in proximal and distal colonic circular smooth muscle from 6- and 30-month-old Fischer rats (n = 5). After mucosa was removed, measurem ents were made on strips stimulated with 80 mm KCl in a 2.5 mm Ca2+ Kr eb's solution. Muscle strips were quick released at peak isometric for ce (F0) to afterloads of 60% of F0. The changes in muscle length from zero to 40 msec and 1 to 2 sec after release during isotonic contracti on were used to calculate the SEC and V0 as a fraction of total muscle length. Passive force (F(p)) was measured in 2.5 mm Ca2+ Kreb's solut ion and in a zero Ca2+, 0.1 mm EGTA solution to determine the contribu tion of contractile and passive elements to passive force. The results of these studies indicate there is no difference in the V0 (L0/sec) o f adult (8.4 +/- 1.5) and aged (7.5 +/- 2.0) animals (P greater-than-o r-equal-to 0.05). Peak force (F0) in the distal colon of the aged rats was greater than adult rats (1.23 +/- 0.1 vs 0.85 +/- 0.01 kg/cm2, P less-than-or-equal-to 0.05). The stiffness of the parallel elastic com ponent and the length-tension relationship were similar in adult and a ged animals. Negligible decreases in F(p) were observed in zero calciu m medium. However, basal contractile tone was elevated in aged animals (P less-than-or-equal-to 0.05). These studies indicate basic differen ces in aged colonic circular muscle that may contribute to altered bow el transit and function during ageing.